Immersed in a Korean challenge

W Janssen, managing director of Engineering Consultants tunnel between the two islands. The gradient would be too great and the slopes too long (TEC), which is technical advisor to Daewoo E&C, and BH Yang, for driver comfort and safety. For this reason managing director of the GK Immersed Tunnel Site for Daewoo E&C, an immersed tube tunnel, just under the seabed, was a logical choice. describe the -Geoje immersed tube tunnel, in South Korea The geological strata varies along the tunnel alignment but top-down typically usan is the second largest city in Link Corporation, a cooperative formed by consists of marine clay, followed by marine South Korea, located in the south- seven Korean contractors led by Daewoo sand, and gravel on top of the bedrock. east of the country and bordered Engineering & Construction, was awarded Marine clay forms the seabed along the Bby the Korean Strait to the south the concession to design, construct and tunnel alignment except the shore areas and mountains to the North. The new operate the Link for a period of 40 years. A (figure 3) where outcrops of bedrock and Busan-Geoje Fixed Link is part of a dual- JV of Tunnel Engineering Consultants (TEC) shallow sand and gravel layers are found. carriage highway that will improve and Halcrow are providing technical advice. The thickness of the marine clay exceeds connections to Geoje Island, crossing Design of the permanent works is now 20m along most of the alignment and locally navigation channels and connecting small complete and construction of the permanent reaches a thickness of 30m. Most of the archipelago islands to reduce the current 2.5 works has begun, with the last of the first tunnel will be founded on this layer. hour drive from Busan to just 45 minutes. batch of four elements having been The clay typically comprises consolidated The principle components of the Link are immersed in May 2008. to slightly over-consolidated soft structured two cable stayed bridges and a 3240m long The Link crosses three navigation clays. The clay is characterised by internal immersed tube tunnel with two-lane traffic channels: The main channel between Gaduk chalk compounds, which means it acts tubes in each direction. The tunnel has a and Jungjuk islands, with a width of 1800m, relatively stiff under low stresses but very number of distinctive features, including its a depth of 18m and no height restrictions; soft when the chalk compounds are broken. long length, its depth of over 50m, its and two secondary channels located Plasticity varies from very to extremely high, offshore location, aggressive marine between Jungjuk-Jeo islands and Joe-Geoje with an index ranging from 56% to 85% with conditions, soft subsoil and geographical islands, which have a minimum width of an average of 68%. The saturated unit alignment constraints. These features, 435m and two times 202m, with clearance weight of marine clay is 13.9kN/m 3- combined with the overall scale of the heights of 52m and 36m respectively. The 15.4kN/m 3, with a mean value of 14.6kN/m 3. project, make the design and construction of water depth for both secondary channels is The channel is exposed to the Pacific this tunnel a major challenge. It is expected 16m. Given the requirements, a tunnel is the Ocean via the Korean Strait and the East the project will extend the use of immersed obvious way to cross the main channel. Sea to the South. This affects the tube technology for major crossings. The relatively steep shores of Jungjuk and marine conditions on site. The maximum Gaduk islands (figures 1 and 2), and the design wave height, Hs, is 9.20m and the The project need for a deep bored tunnel alignment corresponding mean wave period, Tm, is 15 The project has been developed as a Public approximately 25m to 30m below the sec. The principle wave direction, due to Private Partnership (PPP) project. GK Fixed seabed, made it impossible specify a bored typhoons, is south.

Below: Fig 1 – Site location Right: Aerial view of the project

Busan Newport

Gaduk- Island

Geoje-Island

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The current is mainly influenced by the Level (El) tide, which is typically semi-diurnal with a Approach ramp Approach ramp 60 spring tide range of 1.60m and a maximum Jungjuk Island Gaduk Island current of 0.80m/sec at the tunnel alignment. 40 Cut + cover tunnel and Cut + cover tunnel and ventilation building ventilation building The waves in the channel comprise three 20 Immersed tunnel main components: 0 • Locally generated wind waves, mainly from -20 E18 Sea E1 the north-west and north-east during the E17 E2 E3 E16 E9 E8 E7 E6 E5 E4 -40 E10 winter season E15 E11 E14 E13 E12 Marine clay • Deep water generated wind waves, mainly -60 Tunnel top from the south and south-east, during the Marine sand -80 Rock Tunnel bottom summer season -100 • Deep water swell waves, mainly from the 4500 5000 5500 6000 6500 7000 7500 8000 8500 south and south-east Station (m) Marine works have to account for swell waves with a Hs of more then 0.30m and a Right: Fig 3 - Geological profile period of >6 seconds. In the summer season Above: Cross section of the elements waves exceed these values most of the time. Seismicity in South Korea is mainly a number of alternatives, varying from Sand governed by the Tsushima offshore and the Compaction Piles (SCP), soil replacement, Yangsan onshore fault systems, located in preloading and deep cement mixing, Sand the depression between the Pohang Bay Compaction Piles for the western side and and Busan. However, only a few major soil replacement for the eastern side were events have been recorded. This explains selected as the most technically appropriate why, on a large-scale basis, seismic hazard and cost effective methods. These areas analyses lead to a ‘low hazard’ rating for extend over a considerable distance at both immersed tunnels in Western Europe is Korea. The closest fault to the project is the sides of the tunnel in order to support the limited to a water depth of about 15m. The Yangsan onshore fault and the decisive sub-sea embankment, which rises about deepest currently is the Caland tunnel, in (characteristic) earthquake will be an event 16m above the original seabed and has to , with 26m water to the underside on the Yangsan Fault at a distance of 5km- protect the tunnel against grounding ships of structure. The Bosphorus immersed tube, 10km to the east of the project and with a and erosion. in , will locally have almost 60m. moment magnitude of 5.7-6. The SCP area was pre-loaded with rock to The deep alignment has an impact on the accelerate settlement of the subsoil and immersion process and provisions to Features of the tunnel minimise settlement after placing the tunnel prevent water ingress in the tunnel. However, The Busan-Geoje immersed tube is unique elements. In addition to the SCP piles at this despite the fact that past experience is in a number of ways, posing several location, it became clear later in the design limited to a water depth of 26m, the concept challenges. From its deepest point, the process that the marine clay had to be remains technically feasible. highway’s alignment climbs 95m to its improved over almost the total length of the The elements have been designed so that highest elevation, on the single span cable alignment, for which the use of Deep a defined part of the cross section is under stayed bridge over the middle navigation Cement Mixed (DCM) piles was selected. compression ensuring a sufficient barrier channel. The maximum gradient is 5.2%, The tunnel comprises 18 pre-cast exceeding the maximum design gradient of elements, each about 180m in length and Below: Precast yard and model for 4% set by the Authorities under standard approximately 48,000t in weight. The fabrication of tunnel elements conditions. Due to a depression in the concrete cross section is about 100m 2 with seabed at about 350m east of the western outer dimensions of 26.5m x 9.75m. Two portal, a gradient of 5.2% can only be elements on the Daejuk island side are achieved by positioning the tunnel structure tapered and vary in width from 26.5m to about 8m above the original seabed, this 28.5m to create space for a climbing lane. requires an underwater embankment to The elements are constructed in a cover and protect the tunnel. traditional casting basin, located about 36km Preliminary soil investigations available at from site, which is closed off by a concrete tender indicated a relatively thin thickness of gate structure. The elements are constructed the soft marine clay at the location of the in parallel using a single production line for depression, which would have allowed for each element. The casting facility moves the use of soil improvement in combination along the element length allowing full section with the raised embankment to bury the casting at various locations. The first batch tunnel. More detailed soil investigation of four elements took about 11 months and during design however, showed an the second four were produced in eight extension of the marine clay under the months. The target for each of the remaining depressed seabed. This would require the batches is seven months. vertical alignment to be modified to a deeper At the Jungjuk island side of the alignment position, resulting in a gradient of 6%, which the seabed is about 35m below the average was unacceptable to the Authorities. water level, resulting in 47.5m water to the An extensive study was therefore carried underside of structure, increasing to about out to explore other technical options. From 55m due to wave action. The use of

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of increased settlement because of the Immersion operation underway reduced stiffness in soil behaviour (the ratio between re-compression and compression index is almost 14). In addition, the magnitude of settlement will vary along the alignment due to variations in soil characteristics and amount of backfill. The latter depends on the accuracy of trench dredging, which was expected to be low due of the extreme depths of the tunnel trench and the severe marine conditions. The segmental concrete tunnel has the ability to adjust to limited differential settlements but large joint openings should be avoided. For this reason it was decided to improve the marine clay. At the near-shore ends of the tunnel the marine clay is replaced by gravel. But over the majority of the alignment the clay has been improved by installing Deep Cement Mixing piles. This method involves injecting cement against water ingress through the concrete. At both ends of the tunnel the islands directly into the clay forming 900mm The full cross section is cast in one process have been extended artificially to allow the diameter in-situ columns of clay/cement. avoiding horizontal construction joints and construction of transition zones between the Equipment has been used that allows the hydration cracking due to thermal tensile immersed tunnel and the approaches. installation of four columns at a time, forming stresses in the second cast. Segment joints Hydraulic model tests performed at the a square of 1.80m x 1.80m. Where the tunnel are provided with a double seal: A modified Korean hydraulic institute Kordi showed that alignment is above the original seabed 1.2m- injectable waterstop and an omega (W) to Tetrapods with weights of 50t, 60t and 70t 1.8m diameter Sand Compaction Piles have ensure reliability during an earthquake. are best suited to protect the reclaimed been used. On top of the DCM piles a gravel The trench for the immersed tube was extensions at these locations. bed is formed, on top of which the elements dredged by Van Oord in 2006, in advance of As discussed, the marine clay is the are immersed (figure 4). soil improvement of the marine clay, using a dominant type of soil along the alignment. Its The use of DCM piles also allows for a HAM 316 trailing hopper suction dredger. To thickness varies but usually exceeds 30m gradual change of subsoil stiffness at the achieve the design dredging depth of about and is located directly below the foundation locations where the subsoil in the tunnel 55m, the dredge pipe of the HAM 316 (with a level of the tunnel. This very soft marine clay, alignment changes from marine clay to dredging depth of 40m) had to be modified. with a very high plasticity combined with bedrock at the both ends of the tunnel Hydraulic model tests were carried out at low saturated unit weight, low rate of over- alignment. As such it reduces the differential the DHI laboratory in to investigate consolidation and the structural behaviour of settlement over these areas. the effect of the large waves on the the soil, have been crucial to the foundation A two-level earthquake hazard design permanent tunnel structure. For an extreme method chosen. approach has been adopted. The two event, a typhoon wave of Hs = 9.2m and a Normally, the aggregate equaling the earthquake hazard levels are the Operating return period of 10,000 years was defined. weight of the immersed tunnel, combined Design Earthquake (ODE - return period of From the model tests the tunnel was found with the backfill and rock protection, is less 100 years) and the Maximum Design to be subject to vertical uplift, caused by than the weight of the excavated trench Earthquake (MDE, return period of 1000 wave troughs passing the tunnel and material. Due to this, assuming that the years). In respect of strength, the MDE is horizontal loads caused by the varying water original soil does not settle, minimum regarded as Ultimate Limit State, but in order head over the tunnel cross section. Both construction induced settlement will occur. to survive seismic loads (prevention of major horizontal and vertical forces have been On this basis, immersed tunnels are not failure and maintaining safety) the MDE is investigated. They increase when the grain usually provided with piled foundations. regarded as Service Limit State, with the size of the backfill material decreases. These The Busan situation is special in this forces are however dynamic: Change in respect. Due to the offshore conditions, the direction and intensity cause small backfill material needs to have a higher unit movements of the tunnel element allowing weight than the marine clay in order to: (i) water pressure to balance around the tunnel. Resist the uplift forces caused by wave Where the tunnel protrudes above the action; and (ii) lock in the tunnel horizontally. original seabed large waves will have an This results in an increase of effective stress impact on the stability of the rock-protection. under the backfill and causes settlement of Hydraulic model tests have shown that pre- the backfill and the tunnel. The increase in cast artificial rock elements of more then 30t effective stress could well be in the range of are needed. In order to reduce the weight the over consolidation level, implying the risk and thickness of this layer Accropode- and Core-loc blocks have been chosen for the Right: Offshore equipment for making most affected part of the tunnel. SCP piles up to 70m depth

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1 Sea Above: Fig 4 – Cross section DCM piles 4 1 requirement that all joints shall remain 2 watertight and rebar stress does not exceed yield strength - fyk. Peak Ground Acceleration (PGA) of DCM piles Marine clay 0.154g for the MDE event is specified in the Korea codes. Design Peak Ground Velocity A B C D E F G H I J (PGV), and Peak Ground Displacement (PGD) of rock outcrop motions, based on extension along the alignment of the tunnel); Conclusion the specified PGA and the characteristics of snacking (longitudinal vertical and horizontal The Busan-Geoje immersed tube tunnel is the controlling earthquakes, is in the range curvature); and racking (distortion of the unique for many reasons. Its features go of PGV: 8-10cm/sec and PGD of 2-3cm. cross section). well beyond common concrete immersed Free-field soil motions at discrete elevations The first and second responses were tunnel technology. Not all of the special of the tunnel elements are determined using analysed as longitudinal models, in which design aspects were identified in depth at SHAKE. Bedrock ground motions can be the tunnel segments have been modelled as the beginning of the project, but awareness accelerated to 0.35g and the maximum free- frame elements supported in three developed during the design process. field displacements (PGD) at rock and tube directions by soil springs. Free field At the time of writing this paper, the level are 2.9cm and 5.5cm, respectively. displacements at tunnel level were applied design stage had been completed and Ground movement along the tunnel to the soil springs. construction was well on its way. The first alignment has been determined for each The racking was analysed using a 2D four elements have been successfully direction for various displacement time finite element model. The models were used immersed in the tunnel trench and the next histories. The soil structure interaction has to estimate the openings of the joints and batch is waiting for immersion, expected in been divided into three types of tunnel forces in the structure and shear keys during October 2008. The completion of the Link is responses: Worming (axial compression and seismic event. expected in 2010. T&T

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